Washington has already branded Nicolas Maduro a 'dictator' and Trump lists 'military option' as one of many.

US President Donald Trump has threatened military intervention in Venezuela, a surprise escalation of Washington's response to Venezuela's political crisis that Caracas disparaged as "craziness".

Venezuela has appeared to slide toward a more volatile stage of unrest in recent days, with anti-government forces looting weapons from a military base after a new legislative body usurped the authority of the opposition-controlled congress.

Washington has slapped sanctions on President Nicolas Maduro and some of his allies, and branded him a "dictator" over his attempts to crush his country's opposition. Venezuela has in turn accused America of "imperialist aggression".

"The people are suffering and they are dying. We have many options for Venezuela including a possible military option if necessary," Trump told reporters in an impromptu question and answer session.

The comments appeared to shock Caracas, with Venezuela's Defense Minister Vladimir Padrino calling the threat "an act of craziness".

The White House later said Venezuelan President Nicolas Maduro requested a phone call with Trump on Friday, which the White House appeared to spurn, saying in a statement that Trump would gladly speak to Venezuela's leader when democracy was restored in that country.

Venezuelan authorities have long said US officials were planning an invasion.

A former military general told Reuters news agency earlier this year that some anti-aircraft missiles had been placed along the country's coast for precisely that eventuality.

In Washington, the Pentagon said the US military was ready to support efforts to protect US citizens and America's national interests, but that insinuations by Caracas of a planned US invasion were "baseless".